This invention relates generally to devices for monitoring the voltage of a direct current source, such as a storage battery or the like. This invention is particularly adaptable to monitor battery activated systems such as the ignition systems of automobiles, trucks and boats.
Furthermore the novel circuit of the invention is admirably suited to control other equipment as the voltage under monitor passes through a preset value. As an example, the invention is well suited to monitor the voltage of a battery and also to control a generator or alternator which is used to charge the battery.
A particular novel and useful feature of the circuit of the invention is that it uses only one reference voltage. Furthermore the invention uses only one set point voltage derived from the battery. The reference voltage of the invention is taken from a voltage reference source such as a zener diode, or from a stabilized voltage source macrocircuit the reference voltage of which is not sensitive to temperature.
In contrast, the voltage monitors of the prior art have been known to demonstrate temperature sensitivity, which is detrimental in the automotive environment where automobiles may be subjected to extremely low temperatures during the winter. Also these prior art monitors suffer from excessive current drain and, in some cases, require switching in order to compare the battery voltage with a reference voltage. Some prior art voltage monitors require a thermistor network to compensate for fluctuations of component values over a range of temperature. Further prior art voltage monitors employ incandescent bulbs and alarm bells, all of which draw large currents and make continuous monitoring of the battery voltage level impracticable. In contrast, the present invention presents a low current drain to the battery and therefore allows continuous monitoring of the battery voltage.
Other prior art voltage monitors teach two reference voltages and two set point voltages, where other prior art teaches only one reference voltage but two set point voltages. Two reference voltages or two set point voltages make adjustment of a voltage monitor very difficult. In contrast, the present invention teaches only one reference voltage and one set point voltage. The present invention cleverly combines the novel features of one reference voltage, one set point voltage, and all power for the device being drawn from the voltage source being monitored.